Road Design

The Romans built roads to last. In fact they were like massive walls, one metre high, sunk into the soil with the top providing the road surface. ideally, it consisted of four layers: slabs embedded in mortar formed the foundation; masonry made up the second; the third, called the nucleus, consisted of agglomerates; finally, the rolling surface could be simply broken stones, paving stones, or bricks.

With the collapse of the Roman Empire this or in fact any technology was lost for more than a thousand years. The absence of durable roads only really began to be felt with the increase in numbers and size of wheeled traffic during the seventeenth century following a rapid expansion of trade. At first vehicles were seen as a nuisance by those unfortunates made responsible for roads but without funds or knowledge to carry it out. Having tried unsuccessfully to minimise wear and tear by use of controls on weight, axle width, number of horses used, and width of wheels, all of which were easily evaded, serious attempts began during the middle of the 18th century to design roads that would accomodate the traffic.

People disagreed about how roads should be constructed. Some proposed that they should be concave, or even placed in a trench and periodically flushed out. The more enlightened suggested they be sloped across their width. However, certain basic principles began to be enunciated, notably the most important, that of sound drainage. Around 1750 Tressaguet in France and Metcalfe in the UK proposed a method of construction relying on a firm well-drained foundation of large rocks topped by progressively smaller ones, forming a convex surface to make it more impervious to water.

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This proved extremely strong but also highly expensive to build and maintain. Around the end of the eighteenth century Telford in the UK proposed a similarly robust concept, perhaps too much so for traffic needs (he had been looking forward eagerly to steam-powered vehicles), with the result that it was rather expensive due mainly to its thickness and very solid foundation, intended to compensate for unstable roadbeds.

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